Update install-kubectl.md

For install of kubectl on debian based distros, also install gnupg2 for apt-key add to work

content/en/docs/tasks/tools/install-kubectl.md

@@ -59,7 +59,7 @@ You must use a kubectl version that is within one minor version difference of yo
 
 {{< tabs name="kubectl_install" >}}
 {{< tab name="Ubuntu, Debian or HypriotOS" codelang="bash" >}}
-sudo apt-get update && sudo apt-get install -y apt-transport-https
+sudo apt-get update && sudo apt-get install -y apt-transport-https gnupg2
 curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -
 echo "deb https://apt.kubernetes.io/ kubernetes-xenial main" | sudo tee -a /etc/apt/sources.list.d/kubernetes.list
 sudo apt-get update